Description

MEMBRANES Plasma membrane is an envelop surrounding the cell. Separates & protects the cell from the external hostile environment. Besides being a protective barrier, PM provides a connecting system b/n the cell & its environment.

Structure of membranes Lipid bilayer model-- Davson&Danielle(35) Fluid mosaic model-Singer&Nicolson(1972 FM model is more recent and acceptable Thickness---5-8nm A membrane is essentially composed of a lipid bilayer. The hydrophobic (non-polar) regions face each other at the core of the bilayer while the hydrophilic (polar) regions fact outward Globular proteins are irregularly embedded in the lipid bilayer.

Passive diffusion A simple process which depends on the concentration gradient of a particular substance across the membrane. Passage of water & gases through the membrane occurs by passive diffusion. The process does not require energy.

Facilitated diffusion This is somewhat comparable with PD, since the solute moves along the concentration gradient (higher to lower) and no E is needed. FD occurs through the mediation of carrier or transport proteins. Specific carrier proteins for the tpt of glu, gal, leu, phe etc have been isolated and characterized.

Factors affecting rate of diffusion 1. Permeability of the cell membrane 2. Temparature 3. Conc.gr / electrical gr of the substance across the cell membrane 4. Solubility of the substance 5. Thickness of the cell membrane 6. Size of the molecules 7. Size of the ions. 8. Charge of the ions

Active transport AT occurs against a conc. gradient & this is dependent on the supply of metabolic E (ATP) This is also a carrier mediated process like FD. Movement of substances against the chemical or electrical or electrochemical gradient is called AT. It is like swimming in the opp direction of water flow in a river( also called uphill transport) The E required is liberated mostly by break down of high E compounds like ATP.

Active transport AT occurs with the help of carrier proteins as in the case of FD. (mech .is diff) Each CP can carry only one substance or more than one across the membrane. Those CPs transporting only 1 substance are called uniports/uniport pumps. Those CPs carrying more than one substance are called symports/ antiports

Mechanism of active transport When a substance to be transported across the cell membrane comes near the cell, it combines with the CP of the membrane and a Sub-Pro Complex is formed. Now this SP complex moves towards the inner surface of the membrane. Now the substance is released from the CPs The same CP moves back to outer surface of the membrane to transport another molecule of the substance.

Types of AT 1. Primary AT:- In this type of AT, the E is liberated directly from the break down of ATP. By this method Na+ , K+ , Ca2+ , H+ , Cl- are transported across the membrane 2. Secondary AT:- When Na+ is transported by a CP, another substance is also transported by the same protein simultaneously, either in the same direction or in the opp direction. This type of tpt of a sbustance with Na+ by means of a CP is called the secondary AT. i)CP tpts 2 diff molecules in the same direction (symport) Ii)CP tpts 2 diff molecules in opp.direction (antiport)

Primary active transport of Na+ & K+ (Na+ - K+ Pump) Na+ & K+ are transported across the membrane by means of common mechanism called Na+ -K+ Pump. This tpts Na+ from inside the cell to outside and K+ from outside into the cell. This pump is present in all parts of the body. Cells have a high intracellular K+ conc and a low Na+ conc. This is essentially needed for the survival of the cells. High cellular K+ is needed for the optimal glycolysis & for protein biosynthesis. Further Na+ & K+ gradients across the membranes are needed for the transmission of nerve impulse.

CP of Na+ -K+ Pump The CP involved in Na+ -K+ Pump has got 6 sites 3 receptor sites for Na+ . These are on the inner (towards the cytoplasm) surface of the protein molecule 2 receptor sites for K+ . These are on the outer (towards extra cellular fluid) surface of the protein molecule 1 site for the enzyme ATPase (mol.wt250000) which is near the sites for Na. ATPase consists of 2α & 2β subunits and represented as (αβ)2.

Mechanism of action of Na+ -K+ Pump 3 Na ions from the cell get attached to the receptor sites of Na+ on the inner surface of the CP. 2 K ions outside the cell bind to the receptor sites of K+ located on the outer surface of the CP The binding of Na+ & K+ to the CP immediately activates the ATPase. ATPase causes the break down of ATP to ADP with the release of one high E phosphate.

Mechanism of action of Na+ -K+ Pump Now the E liberated causes some sort of conformational change in the CP. Bcos of this, the outer surface of the CP with K+ , now faces the inner side of the cell. And the inner surface of the CP with Na+ faces the outer side of the cell. Now dissociation and release of the ions takes place so that the Na+ are released out side the cell and the K+ are released inside the cell. The exact mechanisms involved in the dissociation and release of the ions are not yet known.

Electrogenic activity of Na+ -K+ Pump Na+ -K+ Pump moves 3 Na+ outside the cell and 2 K+ inside the cell. Thus, when the pump works once, there is a net loss of one +vely charged ion from the cell. The continuous activity of these Na+ -K+ Pump causes reduction in the no.of +vely charged ions inside the cell. Leading to the development of –ve potential inside the cell. This is called electrogenic activity of Na+ -K+ Pump.

Natrium-Kalium-Pumpe - Biologie-Schule.de

The Sodium-Potassium Pump - Georgia State University

The sodium-potassium pump moves toward an equilibrium state with the relative concentrations of Na + and K + shown at left.Read more

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